Microcline twinning This photo is what you see when looking at a thin section of the mineral microcl
Microcline twinningThis photo is what you see when looking at a thin section of the mineral microcline through a microscope under cross-polarized light. The grain itself is probably about a millimeter wide.In this setup, light is passed through a polarizer, sent through a very thin slice of the mineral, and finally passed through one more polarizer before it reaches the eye/camera. If the mineral structure was continuous, the light would pass through unless the mineral structure was perfectly aligned with the path of the polarization of the light– in that case the light would be blocked by the combination of polarizers. It would either be fully bright or fully dark.Microcline is a potassium-rich feldspar. As you can see, it does something very interesting structurally. When it forms, it has a continuous crystal structure, but when it cools down, that structure breaks down along perpendicular planes. This process creates a pattern we call “twinning”, where some parts of the grain are dark in a microscope and adjacent portions are bright.It’s still a single mineral grain, but now the structure of the mineral has been distorted on planes inside the grain.Microcline has a unique feature – it produces this “scotch-plaid” twinning pattern, with two twinning directions about 90 degrees apart. The pattern winds up being plaid, or crosshatched, as you see here. Light is being blocked by some portions of the mineral structure and let through by adjacent portions, creating the on-off pattern.The pattern of light is giving you a window to the arrangement and bending of molecular bonds inside of the mineral.-JBBImage credit: Wikimedia commonshttp://en.wikipedia.org/wiki/File:Microcline.jpg__ -- source link
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